1. I would suggest using a might merc over holy freeze. It's not that holy freeze will turn possible summon corpses into shattered ice, but might just helps out a lot with your skeletal warrior killing power. Might aura also has a longer range from my experience. Clay golem + decrep will definitely be enough to help your skellies take on a boss. Experiment with amp damage too. I would suggest using amp if you find that your skeletons don't really die to bosses. Personally I always use amp damage in every occasion, and my skellies are high enough level so that they don't die to act bosses. Corpse explosion will become one of your best killers. CE paired with amp damage, my summoner can clear groups of monsters very fast.

2. Poison nova isn't as effective in terms of killing speed as CE. Nova will also require a lot of synergizing if it want it to me visibly effective. The cheapest and fastest way to go would still be CE. Adding points to it to increase blast radius is a must if you can spare it later on. Amp dmg, CE, and you'll be clearing crowds very easily.

Dim vison is alway nice to have. You can stop the range attacks from hitting you as you and your army take out the melees.

Poison get higher dmg the bone with less points so that may be a way to go. But there are more poison immunes than magic.

10 in CE with plus to skills in the min. If you go any lower bad things happen.

I also would say might over holy freeze because skeletons are strong sobs and they dont really need the enemy slowed to survive. Also the only people immune to magic revive skillies so you need a high dmg army to break through the bad guys and get to the revivers